In conventional steering systems for a vehicle, a steering wheel is connected via a steering column and a steering rack to one or more vehicle wheels. When the driver applies a rotational motion to the steering column, the motion is transferred via the steering column to a pinion. The pinion converts the rotational motion into translational motion of a steering rack, which moves the vehicle wheels. Hence, the steering wheel, the steering rack and the vehicle wheels are mechanically coupled such that the rotation of the steering wheel, e.g., an angle, a rate of change of the steering angle, and an acceleration of the rotation, uniquely determines the rotation of the vehicle wheels, and vice versa.
In active steering systems, an additional actuator, such as a variable gear ratio including a harmonic motor, provides an additional degree of freedom so that the rotation of the vehicle wheels is not directly determined by the rotation of the steering wheel. Thus, in the active steering systems, an electromechanical actuator can modify the state of the vehicle wheels regardless of the rotation steering wheel, although the rotation of the steering wheel is still controlled by the driver.
In steer-by-wire systems, see e.g., U.S. 20110132682, U.S. 20110276231, there is no mechanical linkage between the steering column and the steering rack. The steering wheel and vehicle wheels coupled to two actuators. One actuator arranged in the steering column controls the rotation of the steering wheel and another actuator arranged in the steering rack controls the rotation of the vehicle wheels. Accordingly, an independent control of the states of the steering wheel and the vehicle wheels is possible. The two actuators are controlled by an electronic control unit that is uses using torque sensors, angle sensors, and angular rate sensors located at the steering wheel and vehicle wheels, and possibly in intermediate positions for control of the steering wheel and the vehicle wheels.
Several methods describe the control of the steer-by-wire systems for the cases when the driver is controlling the vehicle. Specifically, those methods describe that the steering angle of the steering wheel is determined by the driver, and only an appropriate wheels angle is determined, possibly together with a feedback torque on the steering wheel. For example, patent documents U.S. Pat. No. 6,363,305, U.S. Pat. No. 7,908,056, U.S. Pat. No. 7,234,563 describe methods for generating an appropriate feedback torque on the steering wheel, and patent documents US20090048736 and US20030019685 describe methods for controlling the steering rack position and velocity through the steering wheel. Similarly, in U.S. application 20030055546, a method for controlling the rack position based on the driver controlled position of the steering wheel is described. That method also determines a feedback torque on the steering wheel to provide a tactile feedback to the driver.
During a semi-autonomous driving of the vehicle, a semi-autonomous driving planning (SADP) system can assist the driver while the driver still operates the vehicle. For example, the semi-autonomous driving of the vehicle can be used for collision avoidance, stability recovery, lane keeping, see, e.g., patent documents U.S. Pat. No. 8,190,330 and U.S. Pat. No. 8,442,713.
If semi-autonomous driving is used in the vehicles with the steer-by-wire systems, then the SADP determines only a target value for an angle of the vehicle wheels, because the angle of the vehicle wheels in the steer-by-wire system is controlled independently from the steering angle of the steering wheel. However, the steering wheel remains under control of the driver resulting in uncoordinated control of the steering wheel and the vehicle wheels.